1. Field of the Invention
The present invention relates to a special steel and to the manufacture of pressure vessels. Preferably the pressure vessels are adapted to work under pressure under conditions in which there is a risk of H.sub.2 S-induced stress cracking.
2. Discussion of the Related Art
In the petrochemical industry, pressure vessels are used for heating gases having high H.sub.2 S contents. These vessels, which work under pressure and contain inflammable gases, pose significant safety problems which are solved by applying construction rules codified by various standards or construction codes, particularly the NACE MR 0175-97 standard and the codes of the ASME-code type. H.sub.2 S, particularly in the presence of moisture, induces a risk of failure by stress corrosion, and the NACE standard defines H.sub.2 S partial-pressure conditions for which particular construction rules have to be observed in order to guarantee safety of the plants. These construction rules are also defined by the standard and imposed on manufacturers.
In general, the NACE MR 0175-97 standard stipulates that the materials must give satisfactory results when they are subjected to cracking tests in the presence of hydrogen, defined by the NACE TM 0 177-90 standard, and indicates in a very general manner the materials and the operating conditions likely to give satisfaction. In the case of pressure vessels, it is theoretically possible to use carbon or low-alloy steels, both in the normalized state and in the quench-tempered state, as long as they contain less than 1% nickel and have a hardness of less than or equal to 22 HRC. If the vessels and their components were stress-relieved, the stress-relieving operation must have been carried out above 595.degree. C. In addition, after the components have been joined together by welding, the vessels must be subjected to a postweld heat treatment at a temperature of greater than 620.degree. C. so as to obtain a hardness of less than or equal to 22 HRC at any point.
In general, pressure vessels working under conditions in which there is a risk of H.sub.2 S-induced stress cracking are manufactured using carbon and manganese steels in the normalized state, the guaranteed tensile strength R.sub.m of which does not exceed 485 MPa. As a result, the plants thus constructed have a large wall thickness and are therefore very heavy. The heavy weight is a problem, especially in the case of plants installed on offshore platforms.
In order to increase the guaranteed mechanical properties, it has been proposed to use carbon and manganese steels in the quench-tempered state. However, these steels do not allow a tensile strength of greater than 500 MPa to be guaranteed, nor a yield stress of greater than 400 MPa. Likewise, these properties can be guaranteed only in the case of thicknesses not exceeding approximately 80 mm.
It is also possible to use low-carbon steels microalloyed with vanadium or niobium and obtained by controlled rolling. These steels allow a guaranteed tensile strength level of approximately 550 MPa and a guaranteed yield stress level of approximately 450 MPa to be achieved. However, on the one hand these steels cannot be used to manufacture hot-formed components, and on the other hand they can only be used with thicknesses less than 40 mm.
Certainly, there are many low-alloy steels used in boilermaking in the quench-tempered state which allow higher design mechanical properties to be obtained, but these steels do not allow the conditions stipulated by the NACE standard to be met. In addition, they require welding precautions that it is not always easy to reliably comply with on work sites, especially when repair operations are being carried out. The use of these steels for the type of application envisaged here would run the risk of creating defects in the welds, and consequently the risk of serious incidents.
More specifically, in order to manufacture safe pressure vessels, suitable welding conditions must be chosen, these being characterized especially by a minimum preheat temperature and a minimum welding energy per unit length. These welding conditions may be combined in the form of a cooling time between 800.degree. C. and 500.degree. C. of the weld bead or of the zone affected by the welding heat (as defined in the NF: A 36-000 standard). To meet the maximum hardness criterion of 22 HRC, the inventors have found that this cooling time must be greater than a critical value which they call "800/500 cct" (which will be defined more fully later) and which depends on the steel used and on the constraints imposed by the construction codes. The welding is more difficult to carry out reliably the higher this value is. The quench-tempered steels used in boilermaking have an 800/500 cct (critical cooling time between 800.degree. C. and 500.degree. C.) of greater than 10 s, which is too long to allow these steels to be used tinder satisfactory conditions for manufacturing H.sub.2 S-resistant pressure vessels.